Polyphenylene ethers are engineering plastics and, when blended with other resins, provide unique characteristics which make the blends useful in various applications.
Methods for producing polyphenylene ethers by oxidative polymerization of phenols are well known, and the polyphenylene ethers are generally obtained by reacting phenols with oxygen in the presence of a catalyst system composed of a combination of manganese salts or copper salts and certain ligands typified by amines. Methods using manganese salts as a catalyst are described in, for example, Japanese Patent Publication Nos. 21894/1984, 23297/1980 and 31919/1978. Methods using copper salts as a catalyst are described in, for example, Japanese Patent Publication Nos. 23332/1984, 44353/1978 and 34832/1980.
One problem involved in these prior art methods is that during the oxidative polymerization of phenols, diphenoquinone forms as a by-product and this results in coloration of the polymer produced. Therefore, post-treatments such as extraction with solvents and washing are required, leading to increase production costs. Further problems are that since the yield of polyphenylene ether produced per unit amount of the catalyst is low, the expensive catalyst must be used in a large amount and to remove the residual catalyst (e.g., Mn salt or Cu salt) contained in the polymer product of polyphenylene ether, complicated post-treatments are required. In addition, the oxidative polymerization of phenols is an exothermic reaction and the reactor must be equipped with an apparatus for removing the heat of polymerization in the production of polyphenylene ether. Needless to say, higher temperatures of polymerization reaction are advantageous to remove the heat of polymerization. However, it is difficult in the conventional methods to increase the polymerization reaction temperature, and the polymerization reaction temperature of the conventional methods which use manganese salts as a catalyst is 25.degree. to 30.degree. C. Therefore, the conventional methods require not only an expensive apparatus for removing the heat of polymerization but also a complicated polymerization reactor, resulting in increasing the production costs.